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In about 1850, humans began to alter the Estuary on a large scale. These alterations were initiated as tens of thousands of immigrants from all over the world traveled to California in search of riches associated with the discovery of gold. Most of these immigrants arrived by ship, transforming San Francisco from a remote outpost into a teeming frontier city in a matter of a few years.
Over the next century, 90 percent of the Estuary’s wetlands were destroyed, and virtually every major tributary river was dammed and its water diverted. Toxic urban, industrial, and agricultural pollutants were dumped into the Estuary’s waters, and exotic species of plants and animals were released locally, driving out native species. Not surprisingly, the abundant wildlife community that had been supported by the Estuary ecosystem vanished almost entirely. Today, once vast salmon runs are a phantom of their historic antecedents, and populations of endemic wildlife such as the California clapper rail and salt marsh harvest mouse hover on the brink of extinction.
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